How the hell do you call yourself a linguist?
Honestly, do you have a degree in it or something?
The spelling of a word is simply determined by how people naturally spell it.
That's it. The idea that the spelling of words comes about institutionally is an archaic one, and is used only in France and by people with their heads stuck up their ass.
In this case, the plural would most likely be Famitsus.
Lets look at an actual word that had something similar happen to it.
The word "Joseki" came from Japan and it refers to a sreies of plies in a game which individually benefit the players as much as possible.
It has been pluralized as Josekies and Joseki, however, one form is dominant and that would be the "josekis" spelling. Thats just how people naturally spell it, so thats what its spelling is.
The parallelism between this comic and this situation should be clear.
http://www.explosm.net/comics/664/
And this should also apply to your use of bin instead of been.